U.S. Pat. Nos. 3,822,238 and 3,975,350 describe a class of hydrophilic polyurethane polymers which, on contact with an aqueous medium, absorb water with concomitant formation of a stable, water-insoluble hydrogel. In the water swelled state, the polyurethane polymers of the cited patents vary from gel-like to soft and pliable and in the dry state from soft to hard and machinable. Suggested uses are as coatings, linings, dialysis membranes, absorbents, controlled release agents, swellable fabrics, gauzes, solubilizing packaging components, water transmitting coated fabrics, water swelling caulks, wet friction elastomers, artificial leather, gas filters, dentures, hair sprays, nail polishes and oil resistant shapes.
One type of the hydrophilic polyurethanes aforesaid, and one with which the present invention is particularly concerned, is produced by reacting (A) a diol component comprising a water soluble polyoxyethylene glycol having an average molecular weight of from about 400 to about 20,000 and an alkylene glycol selected from the class consisting of ethylene glycol and diethylene glycol, (B) a diisocyanate and (C) water in an amount constituting about 0.3% of the combined weight of the reactants, the ratio of NCO to OH in the diol being about 1:1 or slightly less than 1:1; and the weight ratio of the alkylene glycol to the polyoxyethylene glycol being from about 1:2 to about 1:10.
The resulting polymer is a hydroxy-terminated polyurethane consisting of polyoxyethylene units (--OC.sub.2 H.sub.4)--, so-called soft segments, and ethylene (--C.sub.2 H.sub.4 --) and/or diethylene (--C.sub.2 H.sub.4 OC.sub.2 H.sub.4)--units, so-called hard segments, connected through urethane linkages. Also present in the polymer chain are urea segments which originate from the reaction of the water with isocyanate groups.
In these polymers, hydrophilicity is a function of the soft segments, i.e., the --OC.sub.2 H.sub.4 --units derived from the polyoxyethylene glycol. Thus, to form a more hydrophilic polyurethane, the amount of and/or the molecular weight of the polyoxyethylene glycol reactant is increased to provide more soft segments in the polymer chain. Unfortunately, adding soft segments causes a progressive physical weakening of the polymer as manifested by decreasing tensile strength and tear resistance, particularly when the polymer is in the wet or swollen state.
On the other hand, increasing the hard segments, i.e., the --C.sub.2 H.sub.4 --and/or --C.sub.2 H.sub.4 OC.sub.2 H.sub.4 --units derived from the alkylene glycol, tends to increase the strength of the polymer. However, as more hard segments are introduced, the polymer becomes increasingly rigid and less hydrophilic, resulting in a diminished capacity to form hydrogels when immersed in aqueous media. Another deleterious side effect of adding hard segments is to decrease the solubility of the polymer in organic solvents, such as lower aliphatic alcohols of 1 to 5 carbon atoms, e.g., ethanol and isopropanol.
Nevertheless, by adjusting the proportions of soft and hard segments, it is possible, within limits, to counteract the unwanted side effects of the soft and hard segments, thereby providing polyurethanes that are a workable compromise of hydrophilic properties and mechanical strength.